multi: use new AdditionalEdge interface.

In the previous commit the AdditionalEdge interface was introduced
with both of its implementations `BlindedEdge` and `PrivateEdge`.
In both cases where we append a route either by a blinded route
section or private route hints we now use these new types. In
addition the `PayloadSizeFunc` type is introduced in the
`unifiedEdge` struct. This is necessary to have the payload size
function at hand when searching for a route hence not overshooting
the max sphinx package size of 1300 bytes.

routing/payment_session_test.go

@@ -138,7 +138,7 @@ func TestUpdateAdditionalEdge(t *testing.T) {
 	require.Equal(t, 1, len(policies), "should have 1 edge policy")
 
 	// Check that the policy has been created as expected.
-	policy := policies[0]
+	policy := policies[0].EdgePolicy()
 	require.Equal(t, testChannelID, policy.ChannelID, "channel ID mismatch")
 	require.Equal(t,
 		oldExpiryDelta, policy.TimeLockDelta, "timelock delta mismatch",